1. Field of the Invention
The present invention provides a novel itaconic acid derivative which is useful as a constituting component of a polymer, and also provides a copolymer containing the novel itaconic acid derivative as a constituting component, and an intraocular lens formed from the copolymer, particularly a foldable intraocular lens which can be implanted into eyes in the state of being folded (hereinafter abbreviated as "foldable IOL").
2. Background Information
A number of studies on an acrylic acid monomer as a constituting component of a polymer have been made. However, there have not been so many studies made on the use of an itaconic acid derivative having two carboxyl groups as a monomer. The itaconic acid derivative of the present invention, which is represented by the general formula [1] or [2] and is useful as a polymer component, has such a structural feature that a pyrrolidone group is contained. The itaconic acid derivative having such a feature is a compound which is unknown in the literature as a monomer. As a matter of course, a polymer containing the monomer as a constituting component is also unknown in the literature. ##STR2##
A main object of the present invention is to prepare a novel polymer containing the above-described itaconic acid derivative as a constituting component and to develop a novel material to be applied to intraocular lenses, particularly foldable intraocular lenses.
An intraocular lens (IOL) is used for implanting in place of a crystalline lens removed on cataract surgery. Since IOL was first used in transplanting in 1949, various studies have been made on IOL materials. As there is a great evolution on operation methods, with the advancement of operation methods, demands on the characteristics of IOL have also changed a lot. Recently, with the popularity of the phacoemulsification procedure etc., it is possible to open a very small incision to remove the opaque crystalline lens and finish the operation. Accordingly, demands on characteristics of IOL to be implanted have been changing. For example, characteristics of so-called foldable IOL make it possible to implant an IOL through the small incision in foldable form and open an IOL in lens the capsule. At the same time, various studies have been made on IOL materials. Polymethyl methacrylate (PMA), Silicone, acrylic resin or the like have widely been used an the IOL materials heretofore, while silicone or acrylic resin can be used as foldable IOL materials. A Copolymer of hydroxyethyl methacrylate and methyl methacrylate may also have been used recently. On the other hand, during the studies on these kinds of materials in order to prevent the effect of UV rays on the retina, they can contain an UV absorber such as hydroxybenzophenone, hydroxyphenyl benzotriazole and so on. Moreover, in order to raise biocompatiability and prevent deposition of cells, a polysaccharide such as heparin in coated on the IOL surface in practice.
The itaconic acid derivative of the present invention is characterized by having a pyrrolidone group, and there has been no report on a polymer made up of the itaconic acid derivative. Paying attention to polymers containing a monomer having a pyrrolidone group as a constituting component, particularly polymers used for the purpose of being applied to an intraocular lens, some prior arts are shown below for reference. These prior arts relate to the polymers whose partial constituting component is an acrylic monomer having a pyrrolidone group. Examples of such polymers are a copolymer of methacryloyloxyethyl-2-pyrrolidone and acrylic acid (JP Laid-Open 28705/1992), a polymer polymerized from the monomer which is formed from amidation of acrylic acid and pyrrolidone (JP Laid-Open 43208/1990). a polymer of polyoxyalkylene structure a with pyrrolidone group (JP Laid-open 8218/1990) and so on. As for copolymers formed form three components, there have been reports only an a copolymer of vinyl pyrrolidone, hydroxyethyl methacrylic acid and methyl methacrylate (JP Laid-Open 105250/1978), and a copolymer of methacryloyloxy ethyl-2-pyrrolidone, alkyl methacrylate and fluoroalkyl methacrylate (JP Laid-Open 150197/1993). Incidentally, the main object of these studies is the application on a soft contact lens and not on an IOL. There have been no report especially on the possibility of application of a foldable IOL in the present invention.
A main object of the present invention is to develop an intraocular lens, more particularly materials of a foldable IOL. During the studies on foldable IOL materials, it is necessary to consider refractive index, tensile strength and recovery speed of the materials. For example:
1) When the refractive index is too low the thickness of IOL increases too much. Contrarily when the refractive index is too high, the aberration of peripheral portion becomes large. Accordingly, it is necessary to choose proper refractive index. Specifically, the preferable range is 1.4-1.6.
2) Because forceps are used to implant a foldable IOL, tensile strength bearing their operation is needed. It is an important factor that marks do not remain on the IOL surface after the operation with the forceps.
3) After foldable IOL are implanted, they must be recovered to thee original form in the lens capsule. Recovering speeds vary depending on an operator's custom and competence. The possibility of mechanical invasion of peripheral tissue usually increases with the acceleration of recovering speed. On the other hand, with the slowdown of recovering speed and the elongation of an operation, the possibility of mechanical invasion of peripheral tissue increases. Accordingly, for the foldable IOL, suitable recovering speed is needed.
Moreover, easy formability should be taken into consideration.
Based on these opinions, though various foldable IOL have been used in practice, it is desired to develop an more preferable material.